MALDI is an ionization technique suitable for an analysis of a sample which barely absorbs laser light or one which is easily damaged by laser light (such as proteins). In this technique, a “matrix”, i.e. a substance which easily absorbs laser light and which is easily ionized, is previously mixed in the sample and the obtained mixture is irradiated with laser light to ionize the sample. Normally, the matrix initially added to a sample is in a liquid form, and this matrix incorporates the target substance of the analysis (analyte) contained in the sample. Subsequently, the matrix is dried to obtain crystalline grains containing the analyte. Then, those grains are irradiated with laser light, whereby the analyte are ionized due to interaction among the analyte, matrix and laser light. In recent years, MALDI has been widely applied in the areas of bioscience and others since it enables an analysis of polymer compounds having high molecular weights without significantly fragmenting them, and furthermore, since it is suitable for microanalysis.
With a mass spectrometric method using MALDI, for example, an image showing the intensity distribution of an ion having a certain mass-to-charge ratio on a sample (mass spectrometric image) can be obtained by sufficiently reducing the spot size of the irradiating laser light and changing the relative position of the irradiation point on the sample. Such a system is known as a “mass spectrometric microscope” or “imaging mass microscope”, and is expected to be particularly applied in biochemical, medical and other technical fields in order to obtain information on the distribution of proteins contained in biological cells.
In the case of obtaining a mass spectrometric image using MALDI (i.e. in the case of performing the two-dimensional mass spectrometric imaging), a sample (e.g. a slice of biological tissue) is put on a sample plate for use with a MALDI-MS, and subsequently, a matrix film is formed on the sample plate. This matrix should preferably be put on the sample plate as evenly as possible. Conventional methods for applying the matrix include spraying, dropping or depositing the matrix in a liquid form onto the sample plate.